For the transmission of microwaves it is known to use components in which a dielectric substrate has a metallized lower surface, referred to as a ground plane, and supports on its upper surface one or more microstrips of copper or other highly conductive metal. When a high-frequency signal is applied between the microstrip and the ground plane, the resulting electromagnetic field is carried mainly in the dielectric substrate but is also partly radiated outward so as to cause possible interferences with other equipment. The intensity of this interfering radiation increases progressively with frequency. The wavelength of the signal in the substrate equals its free-space wavelength divided by the square root of the dielectric constant (.epsilon.) and thus varies inversely with that constant. For a given signal frequency, therefore, conductor sections tied to wavelength (e.g. quarter-wave or half-wave sections) will have to be shorter with increasing dielectric constant. This creates structural problems in the realization of microstrip components for superhigh frequencies.
Dielectric materials commonly used for the substrate of microstrip circuits include alumina (.epsilon..apprxeq.10) and certain plastics. A polymeric material of relatively low dielectric constant (.epsilon..apprxeq.2.3) is glass-fiber-reinforced Teflon available under the name Duroid from the Rogers Company, Chandler, Ariz. At a given frequency, the proportion of electromagnetic energy radiated outward increases with lower dielectric constants; thus, if the level of emitted radiation is a criterion, a conventional microstrip circuit using an alumina substrate can be operated at a higher maximum frequency than one whose substrate consists of a plastic material of relatively low dielectric constant. From the structural viewpoint referred to, on the other hand, polymeric materials with dielectric constants between about 2 and 3 are more desirable.
For the foregoing reasons it has heretofore been difficult to design microstrip components for operation at frequencies higher than about 10 GHz. With such high frequencies there is also the problem of proper tuning to establish or maintain the necessary circuit characteristics such as resonant frequency, line impedance and equivalent length, or to correct unavoidable irregularities. The conventional technique involves the placing of conductive blobs on the substrate or on the microstrip; such a procedure is not very convenient and does not enable continuous adjustment or elimination of overcorrections.